The present invention relates to a coated cutting tool insert particularly useful for turning of steel, like low alloyed steels, carbon steels and tough hardened steels, at high cutting speeds.
High performance cutting tools must nowadays possess high wear resistance, high toughness properties and good resistance to plastic deformation. This is particularly so when the cutting operation is carried out at very high cutting speeds and/or at high feed rates when large amount of heat is generated.
Improved resistance to plastic deformation of a cutting insert can be obtained by decreasing the WC grain size and/or by lowering the overall binder phase content, but such changes will simultaneously result in significant loss in the toughness of the insert.
Methods to improve the toughness behaviour by introducing a thick essentially cubic carbide free and binder phase enriched surface zone with a thickness of about 20-40 xcexcm on the inserts by so called gradient sintering techniques are in the art.
However, these methods produce a rather hard cutting edge due to a depletion of binder phase and enrichment of cubic phases along the cutting edge. A hard cutting edge is more prone to chipping. Nevertheless, such carbide inserts with essentially cubic carbide free and binder phase enriched surface zones are extensively used today for machining steel and stainless steel.
There are ways to overcome the problem with edge brittleness by controlling the carbide composition along the cutting edge by employing special sintering techniques or by using certain alloying elements, of which U.S. Pat. No. 5,484,468, U.S. Pat. No. 5,549,980, U.S. Pat. No. 5,729,823 and U.S. Pat. No. 5,643,658 are illustrated.
All these techniques give a binder phase enrichment in the outermost region of the edge. However, inserts produced according to these techniques often obtain micro plastic deformation at the outermost part of the cutting edge. In particular, this often occurs when the machining is carried out at high cutting speeds. A micro plastic deformation of the cutting edge will cause a rapid flank wear and hence a shortened lifetime of the cutting inserts. A further drawback of the above-mentioned techniques is that they are complex and difficult to fully control.
U.S. Pat. No. 5,786,069 and U.S. Pat. No. 5,863,640 disclose coated cutting tool inserts with a binder phase enriched surface zone and a highly W-alloyed binder phase.
The present invention provides a cutting tool insert for machining steel, including a cemented carbide body and a coating, wherein: the cemented carbide body includes WC, 2-10 wt. % of Co, 4-12 wt. % of cubic carbides of metals from groups 4, 5 or 6 of the periodic table, and N in an amount of between 0.9 and 1.7% of the weight of the elements from groups 4 and 5; the cemented carbide body includes a Co-binder phase which is highly alloyed with W, and has a CW-ratio of 0.75-0.90; the cemented carbide body has a surface zone with a thickness of  less than 20 xcexcm, which is binder phase enriched and essentially cubic carbide free; the cemented carbide body has a cutting edge which has a binder phase content which is 0.65-0.75 of the bulk binder phase content, and the binder phase content increases at a constant rate along a line which bisects said cutting edge, until it reaches the bulk binder phase content at a distance between 100 and 300 xcexcm from the cutting edge; and the coating includes a 3-12 xcexcm columnar TiCN layer followed by a 2-12 xcexcm Al2O3 layer, possibly with an outermost 0.5-4 xcexcm TiN layer.
The present invention also provides a method of making a cutting insert comprising a cemented carbide body having a binder phase, with a binder phase enriched surface zone and a binder phase depleted cutting edge, and a coating, including the steps of: forming a powder mixture including WC, 2-10 wt. % Co, 4-12 wt. % of cubic carbides of metals from groups 4, 5 or 6 of the periodic table, the binder phase having a CW-ratio of 0.75-0.90; adding N in an amount of between 0.9 and 1.7% of the weight of the elements from groups 4 and 5; mixing the powder with a pressing agent; milling and spray drying the mixture to a powder material compacting and sintering the powder material at a temperature of 1300-1500xc2x0 C., in a controlled atmosphere of sintering gas at 40-60 mbar followed by cooling; applying post-sintering treatment; and applying a hard, wear resistant coating by CVD or MT-CVD-technique.